We bought a house which has an outside boiler that is inoperable due to the tank freezing. I can repair but filling the boiler with 100 gallons of antifreeze is too expensive.
Sodium chloride would be cheap, but is too corrosive.
Calcium Magnesium Acetate is also cheap (sidewalk deicer), non corrosive, but cannot find much about solubility (partially soluble).
Potassium acetate is soluble, non corrosive, with good freezing point depression; however uncommon. Can it be made cheaply enough for 100 gallons not to freeze at -20F? Vinegar + potassium carbonate.